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Capability Protocols and Stable Extension Points

Concept Position

flowchart TD
  family["Python Programming"] --> program["Python Object-Oriented Programming"]
  program --> module["Module 09: Public APIs, Extension Seams, and Governance"]
  module --> concept["Capability Protocols and Stable Extension Points"]
  concept --> capstone["Capstone pressure point"]
flowchart TD
  problem["Start with the design or failure question"] --> example["Study the worked example and trade-offs"]
  example --> boundary["Name the boundary this page is trying to protect"]
  boundary --> proof["Carry that question into code review or the capstone"]

Read the first diagram as a placement map: this page is one concept inside its parent module, not a detached essay, and the capstone is the pressure test for whether the idea holds. Read the second diagram as the working rhythm for the page: name the problem, study the example, identify the boundary, then carry one review question forward.

Purpose

Design extension seams around the capabilities consumers need rather than around direct access to internal classes and state.

1. Extensions Need Small, Honest Contracts

A plugin or custom adapter usually needs only a few capabilities:

  • evaluate a rule
  • fetch samples
  • publish incidents

Expose those needs as small protocols or abstract interfaces.

2. Capability Beats Intimacy

If an extension must reach into aggregate internals, the seam is not ready. Stable extension points should ask for inputs and return outputs, not mutate hidden state.

3. Version Capability Contracts Carefully

Adding a required method or widening argument semantics changes the extension contract. Treat that as public API evolution, not a local refactor.

4. Composition Roots Choose Concrete Wiring

Extension authors should implement capabilities, while the composition root decides how those implementations join the rest of the system.

Practical Guidelines

  • Expose small protocols for extension capabilities.
  • Keep plugins and custom adapters outside aggregate internals.
  • Treat capability changes as public API changes.
  • Centralize extension wiring in the composition root.

Exercises for Mastery

  1. Define one capability protocol for a replaceable behavior in your system.
  2. Remove one extension path that depends on private state access.
  3. Review one protocol and identify what would count as a breaking change.